Device for detecting magnetic pole of synchronous ac motor, and magnetic pole detecting method therefor

ABSTRACT

A magnetic-pole detecting system for synchronous AC motors and a magnetic-pole detecting method therefor are provided. The magnetic-pole detecting system for synchronous AC motors includes a position detector  10  for detecting the relative position between the moving element  3  and the stator of an AC motor  5 ; a phase-update command unit  52  for making a plurality of currents with different phases flow in coils  3   a  and  3   b  so that the moving element  3  moves to a plurality of stable points; a reversal command unit  56  for making currents flow in the coils  3   a  and  3   b  so that the moving element  3  reverses from the previous direction in which the moving element  3  has been moved by the phase-update command unit  52 ; a current cut-off command unit  54  for cutting off the currents flowing in the coils  3   a  and  3   b  when the position-detector  10  detects a movement of the moving element  3 ; a reversal-determining unit  64  for determining, based on a position-detecting signal from the position-detector  10 , a direction in which the moving element has moved based on the phase-update command unit  52 , and for determining that the determined direction has reversed between the previous and the present instances; and a stable-point-simulator for simulating a stable point by means of the phase of the currents flowing in the moving element  3  when the reversal-determining unit  64  has detected the reversal.

TECHNICAL FIELD

This invention relates to magnetic-pole detecting systems forsynchronous AC motors (referred to as a magnetic-pole detecting system,hereinafter) and magnetic-pole detecting methods therefor, and moreparticularly, to improvement in initial magnetic-pole detection.

BACKGROUND ART

In a synchronous AC motor, magnetic-pole position needs to be detectedin order to determine the phase of the current that flows in each phasecoil. For this purpose, magnetic-pole detecting systems in whichmagnetic-pole position is detected by making each phase current flow ina certain pattern so that the moving element moves to a correspondingstable point are known.

A conventional magnetic-pole detecting system as mentioned above will bedescribed referring to Japanese Laid-Open Patent Publication 1991-89886.According to the patent publication, a magnetic-pole detecting systemdetects initial magnetic-pole position by changing the phases of anα-phase current I_(α) and a β-phase current I_(β)—which flow in ana-phase coil and a β-phase coil that are formed in the moving element ofa 2-phase AC motor—to phases opposite the direction of movement,corresponding to distance traveled by the moving element, so that themoving element moves to a desired stable point.

That is to say, the magnetic-pole position is detected by making theα-phase current I_(α) and the β-phase current I_(β) of equation (4) and(5) described in the foregoing patent publication flow in the α-phasecoil and the β-phase coil, so that a predetermined force F (equation (6)described in the above patent publication) is exerted on the movingelement so as to move the moving element to a stable point where theforce exerted on the moving element is zero.

In such cases, the travel distance ΔX that the moving element moves isgiven by the following equation (equation (8) in the above patentpublication):ΔX=(τ/4−X)/(1+K) wherein

X is the position of the moving element;

τ is the magnetic-pole pitch; and K is a feedback constant when feedbackis implemented.

According to this equation, since the travel distance ΔX of the movingelement can be reduced approximately in inverse proportion to thefeedback constant K, the travel distance ΔA can be shortened by settingthe feedback constant K to a larger value.

In this situation, since the moving element is made to move to thestable point by feeding back the position of the moving element, themoving element becomes oscillatory in the vicinity of the stable point,and this oscillation is intensified by setting the feedback constant toa large value. Therefore, by providing, as set forth in the above patentpublication, a phase compensator, the oscillation is damped bycompensating, in correspondence with the moving speed of the movingelement, the current phases, which vary depending on the moving amountthat the moving element moves.

However, in the conventional magnet-pole detecting system, in order toinhibit the oscillation of the moving element, a speed detector fordetecting the travel speed of the moving element and a phase compensatorfor compensating the current phases in correspondence with the travelspeed of the moving element must be provided, and furthermore the speedfeedback value of the moving element has to be adjusted, and thus therehas been a problem in that the magnetic-pole detecting system iscomplex.

DISCLOSURE OF INVENTION

The present invention is made to address the foregoing issues and has anobject of providing a simplified magnet-pole detecting system and amagnet-pole detecting method therefor that employ an open-loop controlsystem not requiring feedback of the position, the speed, and the likeof the moving element, and moreover that shorten the distance that themoving element travels to a stable point, and damp the oscillation ofthe moving element caused by the control system.

There is provided a magnetic-pole detecting system according to thefirst aspect, including:

-   -   a synchronous AC motor having phase coils either in a moving        element or in a stator;    -   a position-detecting means for generating a position-detecting        signal to detect positional relationship between the moving        element and the stator;    -   a movement-determining means for generating a movement signal        upon determining that the moving element has moved, based on the        position-detecting signal from the position-detecting means;    -   a current command generating means for generating a first        current command signal for making a plurality of different-phase        currents flow so that the moving element moves to a plurality of        stable points, and for generating a second current command        signal that makes currents flow, the currents having a phase        that makes the moving element move in reverse to the direction        in which the moving element has moved based on the first current        command signal;a current controlling means for making the        currents flow in the phase coils, based on the first and the        second current command signals;    -   a current cut-off means for cutting off the currents flowing in        the phase coils, based on the movement signal from the        movement-determining means;    -   a reversal-determining means for determining, based on a        detection value from the position-detecting means, the direction        in which the moving element has moved based on the first current        command signal, and for determining that the detected direction        has reversed between previous and present instances; and    -   a stable-point simulating means for simulating the position of        the moving element by means of the stable point determined by        the phase of the first current command signal when the        reversal-determining means has detected a reversal, or by the        phase prior to the phase at the reversal.

With the magnetic-pole detecting system, the current command generatingmeans generates the first current command signal having a plurality ofdifferent-phase currents so that the moving element moves to a pluralityof stable points; the currents are, based on the first current commandsignal, made to flow into the phase coils by means of the currentcontrolling means; the position-detecting means detects the movement ofthe moving element; and the current cut-off means cuts off the currentsflowing in the phase coils. When the reversal-determining meansdetermines that the direction, in which the moving element has movedbased on the first current command signal, has reversed between theprevious and the present instances, the stable-point simulating meanssimulates the position where the moving element is at a standstill bymeans of the stable point determined by the phase of the first currentcommand signal upon the reversal or by the phase prior to the phase uponthe reversal; therefore, the stable point of the moving element can bedetected by means of a makeup of the open-loop control system withoutfeeding back the position, the speed, or the like of the moving element.

Therefore, in the magnetic-pole detecting system according to thepresent aspect, since, unlike the conventional art, no moving-elementoscillation caused by feedback of a position or the like occurs when themoving element is made to move to a stable point, the oscillation of themoving element can substantially be damped. In addition, since it isdetected that the moving element has moved and then the current cut-offmeans cuts off the currents flowing in the phase coils, in detecting astable point of the moving element, the travel distance of the movingelement can be shortened. Therefore, an effect is demonstrated wherein asimplified magnet-pole detecting system can be obtained.

There is provided a magnetic-pole detecting method according to thesecond aspect, including:

-   -   a first step of making, by means of the current controlling        means, currents with a first phase flow in the phase coils, so        that the moving element moves, based on the first current        command signal, to a first stable point;    -   a second step of cutting off the currents by means of a current        cut-off means, when a movement-determining means detects, based        on the position-detecting signal, a movement of the moving        element;    -   a third step of making, based on the second current command        signal, currents with a second phase flow in the phase coils, by        means of the current controlling means, so that the moving        element reverses from a direction in which the moving element        has moved;    -   a fourth step of cutting off the currents by means of the        current cut-off means when the movement of the moving element is        detected by the movement-determining means;    -   a fifth step of making the phase of the first current command        signal the phase for a second stable point that is different        from the phase for the first stable point, and of making the        currents flow in the phase coils by means of the current        controlling means;    -   a sixth step of determining, based on the position-detecting        signal from the position-detecting means, a direction in which        the moving element moves based on the first current command        signal, and for determining whether or not the detected        direction has reversed between the previous and the present        instances; wherein the first step through the sixth step are        sequentially carried out, and the stable point, determined by        the phase of the first current command signal when the        reversal-determining means has detected a reversal, or by the        phase prior to the phase at the reversal, is simulated as the        position of the moving element by a stable-point simulating        means.

With the magnetic-pole detecting method, a position-detecting meansdetects that the moving element has moved; the current cut-off meanscuts off the currents flowing in the phase coils; and thereversal-determining means detects that the moving direction of themoving element has reversed between the previous and the presentinstances. Accordingly, with the magnetic-pole detecting methodaccording to the present aspect, since, unlike the conventional art, nomoving-element oscillation caused by feedback of a position or the likeoccurs when the moving element is made to move to a stable point, asimplified device that substantially damps the oscillation of the movingelement can be obtained. In addition, since the current cut-off meansimmediately cuts off currents in the moving element, an effect isdemonstrated wherein, in detecting a stable point of the moving element,a travel distance of the moving element can be shortened.

There is provided the magnetic-pole detecting system according to thethird aspect, including: a cut-off signal generating means forgenerating a current cut-off command signal when a predetermined timeperiod elapses from the occurrence of a first or a second currentcommand signal; wherein the current cut-off means cuts off currents inthe phase coils, based on the current cut-off command signal from thecut-off signal generating means.

In the present aspect, “a previous moving direction in thereversal-determining means” denotes an immediately prior movingdirection in which the moving element has been moved by making currentsflow into the phase coils by means of a first current command signal,when the moving element remains still even when the first currentcommand signal makes the currents flow into the phase coils.

With the magnetic-pole detecting system, an effect is demonstratedwherein a position of the moving element can be detected simply evenwhen the moving element halts at a point where attractive force on themoving element is zero.

There is provided the magnetic-pole detecting system according to thefourth aspect, including: a current-maintaining means, provided in placeof the stable-point-simulating means, for inactivating the currentcut-off means based on the determination, by the reversal-determiningmeans, that the reversal has occurred, and for continuing to make thecurrents with the phase at the reversal or currents with the phase priorto the phase at the reversal flow into the phase coils, until the movingelement is determined, based on the detecting signal from theposition-detecting means, to have come to a standstill.

According to the magnetic-pole detecting system, since thecurrent-maintaining means continues to make the currents with the phaseat the reversal, or the phase prior to the phase at the reversal, flowin the phase coils until, based on the position-detecting signal fromthe position-detecting means, the moving element is detected to havecome to a standstill, the moving element is certain to be moved to astable point. As a result, an effect is demonstrated wherein theaccuracy of detecting a stable point of the moving element is raised.

In the magnetic-pole detecting system according to the fifth aspect, thecurrent phase of the first or the second current command signal is aphase at which sinusoidal cogging torque generated in the synchronous ACmotor is approximately zero.

With the magnetic-pole detecting system, an effect is demonstratedwherein the detecting a stable point of the moving element isunsusceptible to the cogging torque generated by the synchronous ACmotor.

There is provided the magnetic-pole detecting system according to thesixth aspect, including:

-   -   a phase-updating means for updating the phase of the first        current command signal in steps of Δθ;    -   a standstill-determining means for generating a standstill        signal upon determining that the moving element has come to a        standstill, based on the position-detecting signal from the        position-detecting means, after the currents are made to flow        into the phase coils by the first current command signal;    -   a first position-calculating means for obtaining a first        position equal to the first stable point, based on the first        phase of the first current command signal upon the occurrence of        the standstill signal;    -   a second position-calculating means for obtaining a second        position equal to the second stable point, based on the second        phase of the first current command signal when the        reversal-determining means has made a determination; and    -   a stable-point calculating means for obtaining, based on the        first and the second positions, the position of the stable        point; wherein the current controlling means maintains the phase        of the movable element when at a standstill, even when the        currents are made to flow in the phase coils, according to the        first current command signal.

Since the magnet-pole detecting system has a means for compensating,based on the first and the second positions, the position of a stablepoint, an effect is demonstrated wherein the stable-point fluctuationcaused by frictional force between the moving element and a stator canbe suppressed.

There is provided the magnetic-pole detecting system according to theseventh aspect, wherein currents generated by the current controllingmeans have a time constant longer than that determined from theresistance component and the inductance component of the AC motor.

With the magnetic-pole detecting system, an effect is demonstratedwherein an inertial travel distance of the moving element can beshortened since currents flowing in the synchronous AC motor risesmoothly, and attractive force on the moving element increasesgradually.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a characteristic curve chart for moving element attractivetorque vs. phase (distance), representing the principle ofmoving-element position detection according to an embodiment of thepresent invention;

FIG. 2 is an overall block diagram of a magnetic-pole detecting systemfor an AC motor, illustrating an embodiment of the present invention;

FIG. 3 is a flowchart illustrating operation of the magnetic-poledetecting system for the AC motor illustrated in FIG. 2;

FIG. 4 is a curve chart representing a relationship between coggingtorque waveform and a stable point for an AC motor according to anotherembodiment of the present invention;

FIG. 5 is a characteristic curve chart representing attractive torqueand static-friction torque vs. phase (distance) for the moving elementaccording to another embodiment of the present invention; and

FIG. 6 is a flowchart illustrating the operation of detecting a stablepoint taking into account the static-friction torque as represented inFIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Explanation of Position-Detecting Principle for Moving Element

A principle for detecting moving-element position by means of amagnetic-pole detecting system according to the present invention willbe described referring to FIG. 1. FIG. 1 is a characteristic curve chartfor moving-element attractive torque vs. phase (distance), representingthe principle of moving-element position detection.

(1) Fundamental Principle

In a synchronous AC motor comprising a moving element having 2-phasecoils—an α-phase coil and a β-phase coil—and a stator, when a currentwith a phase of −nΔθ flows in each of the α-phase coil and the β-phasecoil based on an electric current command signal, with an attractiveforce F acting on the moving element and a stable-point position X forthe moving element, the following equation is obtained:F=A.I _(c)sin(II/2−nΔθ)+B.I _(c)sin(−nΔθ)  (1), whereinA: φ.cos(2IIX/τ+2IIΔX/τ)B: φ.cos(2IIX/τ+2IIΔX/τ+II/2)τ: magnetic-pole pitch (m)

Rearranging equation (1), the attractive force F is expressed as in thefollowing equation:F=φ.I _(c).cos(2IIX/τ+2IIΔX/τ+nΔθ)   (2)

The moving element is presently at a standstill in a position X_(S) asshown in FIG. 1, and in terms of the above equation (2), when the phasenΔθ of the current I_(c) is varied, the curve chart for the attractiveforce F vs. the phase will be as represented in FIG. 1. In other words,when the phase nΔθ is varied in steps of Δθ, the attractive forcechanges from F₀, to F₁, F₂ and so forth.

The positive-direction attractive forces F₀, F₁ and F₂ are exerted onthe moving element until the phase reaches 2Δθ, and the moving elementmoves in the positive direction; however, after the phase reaches 3Δθ, anegative-direction attractive force F₃ is exerted on the moving element,and the moving element reversely moves in the negative direction. It canbe determined that the moving element is present between the phase 2Δθand the phase 3Δθ by detecting this reversal by means of a positionalsignal from the position-detecting system in the moving element; thatis, the position of the moving element can be detected.

However, if this type of position detection is insufficiently accurate,when reversal of the moving-element is detected, by keeping a current ofphase 3Δθ when the reversal occurs, or of phase 2Δθ prior to thereversal, flowing in the α-phase coil and in the β-phase coil until themoving element 3 comes to a standstill so as to shift the moving element3 to position X₁ or X₂, the position of the moving element can bedetected with a high degree of accuracy.

In cases where the moving element is moved in this way, if the maximalvalue ΔX_(max) of the travel distance of the moving element is to befound from the conditions under which the attractive force F is zero,i.e., the above equation (2) is zero, the following equation will hold:2IIX/τ+2IIΔX _(max) /τ+nΔθ=II/2 ΔX _(max)=τ/4−X−(nΔθ/2II)τ  (3)

Consequently, after detecting that the position of the moving element iswithin a predetermined range, the accuracy of moving-element positiondetection can be improved by shifting the moving element by a minimallynecessary distance.

(2) Return of the Moving Element to its Former Position

Keeping the currents of phase nΔθ flowing in the α-phase and the β-phasecoils would not be appropriate, since the moving element would move tothe position where the attractive force F is zero (the stable point),prolonging the travel distance of the moving element. In order to avertthis, when the moving element shifts, the currents that flow in theα-phase and the β-phase coils are cut off, based on a position-detectingsignal from the moving-element position-detecting system, so that theattractive force on the moving element is zero, and the moving elementis kept from shifting.

However, after the moving element has moved, even with the attractiveforce of the moving element equal to zero, the moving element slows withnatural deceleration speed determined by the kinetic frictioncoefficient and the like of the moving element, and comes to astandstill after coasting, for example, for several micrometers. Inorder to address this, after the moving element has moved in reverse,with the currents of phase nΔθ+II being made to flow in the α-phase andthe β-phase coils, by cutting off the currents that flow in the α-phaseand the β-phase coils, based on a position-detecting signal from theposition-detecting system, and by returning the moving element to itsformer position, the halting position of the moving element does notshift.

(3) Position Detection when Moving Element is Halted at Point X_(c) atwhich Attractive Force F is zero.

The foregoing has described a situation wherein the moving element ishalted at positions other than those where the attractive force F iszero. However, on rare occasions, the position (stable point) where theattractive force F of the moving element is zero may coincide with astandstill position of the moving element. For instance, when the movingelement is at a standstill at the position X_(c), the stable point ofthe moving element resulting from the attractive force F₂ coincides withthe position X_(c) of the moving element. Therefore, when the attractiveforce F₂ resulting from the phase 2Δθ occurs, since no attractive forceis exerted on the moving body, leaving the moving body at a standstill,no moving direction for the moving body exists. Therefore, after apredetermined time period following the issuance of a current commandthat makes the currents flow in the α-phase and β-phase coils, e.g.,after 100 ms, the currents in the α-phase and the β-phase coils are cutoff, and then with the phase of the current that flows in the α-phaseand β-phase coils updated by one to a phase of 3Δθ, the moving elementis made to move toward a new stable point X₂. In this way, movement ofthe moving element is ensured.

The determination of a reverse movement of the moving element isimplemented by varying, in steps of ΔΓ, the phase of the currentsflowing in the α-phase and the β-phase coils and determining that themoving element has reversed between the previous instance and thepresent instance. However, in the case of the 2Δθ phase current, sincethe moving element has been at a standstill, there is no previous movingdirection. Therefore, when the moving element has been halted in theprevious instance in this way, the moving direction of the movingelement in the instance before the previous instance is employed as theprevious instance. In other words, a reverse movement is determined bytaking the moving direction of the moving element directly before as themoving direction of the previous instance.

Moreover, since the moving element may also be halted at a position X₀,currents whose nΔθ phase exceeds 2II are made to flow in the α-phase andβ-phase coils, whereby the moving element is made to reverse forcertain.

EMBODIMENT 1

An embodiment of the present invention that implements the foregoingposition-detecting principle for a moving element is set forth referringto FIG. 2. FIG. 2 is an overall block diagram of a magnetic-poledetecting system in a synchronous AC motor.

In FIG. 2, a magnetic-pole detecting system 1 is equipped with a2-phase, linear synchronous AC motor 5 (referred to as an AC motor,hereinafter) that has a stator 2 and comprises a moving element 3 withan α-phase coil 3 a and a β-phase coil 3 b, each phase coil beingarranged facing, and having a predetermined gap with, the stator 2; aposition detector 10 that detects the position of the moving element 3;a microcomputer 20 that generates an electric current command signal formaking currents flow in the AC motor 5 and that executes predeterminedprocesses based on position-detecting signals from the position detector10; a current generating unit 30 that is connected to the output ofmicrocomputer 20 and, based on the current command signal from themicrocomputer 20, that applies or cuts off currents in the coils 3 a and3 b; current control units 32 and 34 that are connected to outputs ofthe current generating unit 30 and that control currents from thecurrent generating unit 30 so that they flow in the coils 3 a and 3 b;and Ts 36 and 38 that detect currents inputted into the current controlunits 32 and 34 and flowing in the coils 3 a and 3 b.

The position detector 10 functions as a position-detecting means fordetecting, as a position-detecting signal, the position of the movingelement 3 relative to the stator 2, and is equipped with a linear scale12 that is horizontally extended across a travel region of the movingelement 3, and that has positional information for detecting that themoving element 3 has moved horizontally; a light-emitting unit 14 a thatis arranged so as to partially surround the linear scale 12 and thatgenerates light by means of a light emitting diode; a light receivingunit 14 b that receives the light through a phototransistor; and aconcave shape detecting unit 14 fixed on the surface of the movingelement 3. In addition, a linear guide 13 is extended in parallel to thestator 2 so that the moving element 3 moves horizontally with respect tothe stator 2.

The microcomputer 20, wherein the output from the detecting unit 14 ofthe position detector 10 is connected to an input interface (referred toas “I/F” hereinafter) 22, comprises a CPU 24, a ROM 27, a RAM 27, and anoutput I/F 28 that is connected to the input of the current generatingunit 30.

An electric current command unit 50 illustrated in the form of a blockdiagram, indicating the functions of the microcomputer 20, is providedwith a phase-update command unit 52 that generates a forward-movementcurrent command signal with different phases (the first current commandsignal) so that the moving element 3 can move to a plurality of stablepoints; a current cut-off command unit 54 that generates a currentcut-off command signal that cuts off currents flowing in the coils 3 aand 3 b; a reversal command unit 56 that generates a reversal-movementcurrent command signal (the second current command signal) for applyingcurrents in the coils 3 a and 3 b so that the moving element 3 reverses,relative to the previous instance based on the forward-movement currentcommand signal from the phase-update command unit 52; and a stable-pointmovement command unit 58 that, when a stable point of the moving element3 is detected to be within a predetermined range, repeatedly generatesthe current command signal with a particular phase until the movingelement 3 comes to a standstill, and is configured in such a way that acommand signal from only one among the phase-update command unit 52, thecurrent cut-off command unit 54, the reversal command unit 56 and thestable-point movement command unit 58 is given to the current generatingunit 30.

In addition, a current-command means comprises the phase-update commandunit 52 and the reversal command unit 56; a current control meanscomprises the current generating unit 30 and the current control units36 and 38; a current cut-off means comprises the current cut-off commandunit 54 and the current generation unit 30; a current continuation meanscomprises the stable-point movement command unit 58, the currentgenerating unit 30 and the current control units 36 and 38.

A position-determining unit 60 illustrated in the form of a blockdiagram, indicating functions of the microcomputer 20, is provided witha movement-determining unit 62 (a movement-determining means and astandstill-determining means) that, based on a predetermined change in aposition-detecting signal originating from the position detector 10,generates a movement signal as the moving element 3 moves from astandstill, and that generates a standstill signal as the moving element3 comes to a standstill from a movement; and a reverse-movementdetermining unit 64 (a reverse movement-determining means) thatdetermines that the moving element 3 has reversed between the previousand the present movements and generates a reverse signal based on thedetermination.

The reverse-movement determining unit 64 determines that the movingelement 3 has reversed between the previous and present instances, bydetermining, based on a position-detecting signal from the positiondetector 10, the direction in which the moving element 3 has moved basedon a normal-movement-current command signal from the phase-updatecommand unit 52. More concretely, using the fact that, when the movingelement 3 moves in the right-hand direction in FIG. 1, the positionalvalue of the moving element 3 increases based on the position-detectingsignal, and when the moving element 3 moves in the left-hand directionin FIG. 1, the positional value of the moving element 3 decreases basedon the position-detecting signal, it is determined that the movingelement 3 has moved forward when the difference between the previous andthe present positional values increases, and it is determined that themoving element 3 has reversed between the previous and the currentmovements when the difference decreases. Moreover, the reverse-movementdetermining unit 64, in cases wherein the moving element 3 has been at astandstill at the previous instance, determines a reverse movement byregarding the direction in which the moving element 3 has movedimmediately before as the previous moving direction.

In addition, the reverse-movement determining unit 64 is arranged so asnot to detect as a reverse movement a case where the moving element hasreversed based on a reverse-movement current command signal from thereverse movement command unit 56. This is because the reversal in suchcases is not related to detecting the position of the moving element 3.

Assuming Δθ as a current phase, and n as the number of operations thatshift the moving element 3 in a certain direction, e.g., that the numberof operations in the right-hand direction in FIG. 1 is n, thephase-update command unit 52 generates the forward-movement currentcommand signal, while updating the phase in steps of Δθ, so as to makecurrents of phase −nΔθ (n is a positive integer) flow in the coils 3 aand 3 b. In this way, the configuration enables a plurality of differentphase currents to flow, so that the moving element 3 can move to astable point corresponding to the current phase −nΔθ, in other words, sothat the moving element 3 can move to a plurality of stable points.

Such a phase, Δθ, indicates the detection accuracy for the stable pointsof the moving element 3, and if the load angle of the AC motor 5 is, forexample, a maximal value ±10 degrees, the phase Δθ is set as 10 degrees.If currents that flow, based on the forward-movement current commandsignal from the phase-update command unit 52, in the coils 3 a and 3 bare indicated with I_(α) and I_(β), respectively and if I_(c) is themaximal current of I_(α) and I_(β) in their steady states, the followingequations are obtained:I _(α) =I _(c)(1−ε^(−ct)).cos(−nΔθ)   (4)I _(β) =I _(c)(1−ε^(−ct)).sin(−nΔθ)   (5), wherein

n: the number of operationss of the moving element in a certaindirection

c: time constant (second)

t: time period (second).

The reason why the first-order lag current I_(c) (1−ε^(−ct)), which haspredetermined gradient over a time period as in the above equations (1)and (2), is made to flow in the coils 3 a and 3 b is to limit thedistance that the moving element 3 inertially travels, between the timethe moving element 3 starts moving and the time the currents flowing inthe moving element 3 are cut off. In other words, the currents grow insuch a manner that they have a time constant longer than that determinedby the resistance component and the inductance component of the AC motor5; and, when a step voltage is applied to the AC motor 5, the current isdelayed by a time constant, for example, of the order of severalmicroseconds, and the delay is prolonged by a time constant of the orderof several milliseconds, by making the currents with the first-order-lagflow in the coils 3 a and 3 b.

The current cut-off command unit 54 is configured so as to generate theabove current cut-off command signal by detecting, based on theposition-detecting signal from the position detector 10, that the movingelement 3 has moved according to the current command signal (includingboth forward-movement current command signal and reverse-movementcurrent command signal), or that the moving element 3 has not moved evenwhen the predetermined time has elapsed after the issuance of thecurrent command signal.

The reverse movement command unit 56 generates a reverse-movementcurrent command signal so that the currents flow in the coils 3 a and 3b of the moving element 3 according to the equations below, and, basedon this reverse-movement current command signal, returns the movingelement 3 to its initial position by moving the moving element 3opposite to the previous direction of movement. This is because theexact position of the moving element 3 cannot be detected when themoving element 3 moves.I _(α) =I _(c)(1−ε^(−ct)).cos(−nΔθ+II)   (6)I _(β) =I _(c)(1−ε^(−ct)).sin(−nΔθ+II)   (7)

The stable-point movement command unit 58 inactivates the currentcut-off command unit 54 based on the determination by thereverse-movement determining unit 64 that the moving element 3 hasreversed, and continues to generate a current command signal until it isdetected, based on the position-detecting signal from the positiondetector 10, that the moving element 3 has come to a standstill, bygenerating, for the coils 3 a and 3 b, a current command signal of phasenΔθ, which occurs with reverse movement, or of phase −(n−1) Δθ, whichoccurs one step before that at which reverse movement occurs, andconveying this current command signal to the current generating unit 30.

The operation of the magnetic-pole detecting system configured as setforth above is described according to FIG. 1 through FIG. 3.

In a situation wherein the moving element 3 is at a standstill at aposition X_(s) in FIG. 1, when the forward-movement current commandsignal with the phase of −Δθ, and 1 as the number of operations n, isconveyed from the phase-update command unit 52 (to the AC motor 5: inthe original draft, this phrase seems to be inserted by mistake) to thecurrent generating unit 30, the current generating unit 30, based onthis current command signal and by way of the current control units 32and 34, makes the currents I_(α) and I_(β) with the phase of −Δθ to flowin the coils 3 a and 3 b of the moving element 3, so that an attractiveforce Fi is exerted (Step S100, the first step). In this way, the movingelement 3 gradually starts moving slowly toward the first stable point(Step S102).

When the position detector 10 conveys, as the position-detecting signal,the fact that the moving element 3 has moved, to the CPU 24 by way ofthe input I/F 22, the movement-determining unit 62 detects that themoving element 3 has moved, and conveys a determination signal to thecurrent cut-off command unit 54. The current cut-off command unit 54conveys to the current generating unit 30 the current cut-off commandsignal that cuts off the currents Is, and IB flowing in the coils 3 aand 3 b, and then the current generating unit 30 immediately cuts off,by way of the current control units 32 and 34, the currents flowing inthe coils 3 a and 3 b (Step S104, the second step). When the currents inthe coils 3 a and 3 b are cut off, the moving element 3 decelerates atthe natural-deceleration speed determined by its kinetic frictioncoefficient and the like, inertially moves, and comes to a standstill.

Although the reverse-movement determining unit 64 determines, based onthe position-detecting signal from the position detector 10, if thedirection of the moving element 3 has reversed (Step S106), since themoving element 3 is moving for the first time, it is not reversing.Next, in order to return the moving element 3 to its initial position bymoving it in the opposite direction to that in which it has slightlymoved as described above, the reverse-movement command unit 56 conveys areverse-movement current command signal from phase −Δθ+II to the currentgenerating unit 30, and the current generating unit 30, by way of thecurrent control units 32 and 34, makes currents I_(α) and I_(β) of phase−Δθ+II flow in the coils 3 a and 3 b of the moving element 3 (Step S108,the third step). Due to this, the moving element 3 starts moving towardthe stable point determined by the currents I_(α) and I_(β) of phase−Δθ+II, and reverses relative to the movement direction of the movingelement 3 in Step S100. In addition, in this reverse-movement, thereverse-movement determining unit 64 does not determine that the movingelement 3 has reversed. This is because this is a reverse-movement basedon the reverse-movement current command signal from the reverse-movementcommand unit 56.

Upon the reverse-movement, when a position-detecting signal from theposition detector 10 is conveyed to the CPU 24 through the input I/F 22,the current cut-off command unit 54 conveys to the current generatingunit 30 the current cut-off command signal that cuts off the currentsI_(α) and I_(β), and the current generating unit 30 cuts off, by way ofthe current control units 32 and 34, the currents flowing in the coils 3a and 3 b (S112, the fourth step). The CPU 24 adds +1 to the number ofoperations n, and sets the new number of operation to “2” (Step S114).When Step S100 is executed through the forward-movement current commandsignal for the current of phase −2Δθ, an attractive force F₂ isgenerated in the moving element 3, and the moving element 3 moves, butdoes not reverse (Step S102, S104 and S106). Accordingly, the foregoingSteps S108 through 112 are executed, and the number of operations n isincremented by +1 to “3” (Step S114).

When Step S100 is executed through the forward-movement current commandsignal for the current of phase −3Δθ, the current generating unit 30makes the currents I_(α) and I_(β) of phase −3Δθ flow in the coils 3 aand 3 b, so that an attractive force F₃ is exerted. T he moving element3 is moved by this attractive force F₃ toward the second stable point,and the direction of the moving element 3 reverses (Step S102 and S104,the fifth step).

The reverse-movement determining unit 64, based on a position-detectingsignal from the position detector 10, determines that the movementdirection of the moving element 3 has reversed between the previous andthe present movements, since the positional value of the moving element3 has changed from increasing to decreasing, and it generates areverse-movement signal (Step S106, the sixth step); when thereverse-movement signal is conveyed to the stable-point movement commandunit 58, the stable-point movement command unit 58 inactivates thecurrent cut-off command unit 54, and generates a current command signalthat makes the currents of the reverse-movement phases −3Δθ or −2Δθ flowin the coils 3 a and 3 b and conveys the current command signal to thecurrent generating unit 30; and the current generating unit 30 makes thecurrents I_(α) and I_(β) of phase −3Δθ or −2Δθ flow in the coils 3 a and3 b, so that an attractive force is exerted in the moving element 3, andthe moving element 3 is made to move to the position X₂ (Step S116).Here, the moving element 3 is moved to the stable point for the purposeof improving detecting accuracy.

Next, the movement-determining unit 62 conveys to the current generatingunit 30, as zero, a current command signal (a current cut-off commandsignal), which is issued by the stable-point movement command unit 58according to the standstill signal by which it is determined, based onthe position-detecting signal from the position detector 10, that themoving element 3 has halted. The current generating unit 30 cuts off thecurrents flowing in the coils 3 a and 3 b (Step S118); in Step S116, bymaking the currents I_(α) and I_(β), of phase −3Δθ (−2Δθ), to flow inthe coils 3 a and 3 b, the magnetic-pole positions of the moving element3 are stored in a RAM 48 as 3Δθ (2Δθ) and the step is completed (StepS120). Thus, the AC motor 5 is driven with the phase 3Δθ (2Δθ), whichhas been stored in the RAM 27, shifted by II/2.

Moreover, although the above embodiment has been described based on thepremise that the position where the attractive force F generated in themoving element 3 is zero (a stable point) does not coincide with thestandstill position of the moving element 3, on rare occasions theposition where the attractive force F of the moving element 3 is zerocoincides with the standstill position of the moving element 3. That isto say, the moving element 3 may be present at the position X_(c) asshown in FIG. 1.

Since, when the moving element 3 is at a standstill in this type ofposition X_(c), the moving element 3 is not moved by the attractiveforce F₂, the current cut-off command unit 54 generates a currentcut-off command signal after a predetermined time period has elapsedfrom the issuance of the current command signal, and conveys it to thecurrent generating unit 30 (Step S102 and S110), and then the currentgenerating unit 30 immediately cuts off the currents flowing in thecoils 3 a and 3 b by way of the current control units 32 and 34 (StepS104 and S112). Consequently, in Step S114, the number of operations nis incremented by one to “3,” and currents of phase −3Δθ are made toflow, by the phase-update command unit 52, in the coils 3 a and 3 b, sothat the moving element 3 moves in a reverse direction (Step S100through S104). In Step S106, the reverse-movement determining unit 64determines reverse-movement according to the previous and the currenttravel directions of the moving element 3; however, since the movingelement was at a halt at the previous instance, there is no previousmovement direction. Therefore, using the movement direction of theprevious instance when the moving element 3 was at a standstill, i.e.,the immediate previous movement direction as the previous movementdirection, together with the current movement direction,reverse-movement is determined, and a reverse-movement signal isgenerated.

Moreover, in the above embodiment, the stable-point movement commandunit 58 generates in the coils 3 a and 3 b a reverse-movement currentcommand signal that results in the current of phase −nΔθ, which is thephase at reverse-movement, and conveys it to the current generating unit30; and a command is issued to the phase-update command unit 52 to keepgenerating the reverse-movement current command signal until it isdetermined, based on the position-detecting signal from the positiondetector 10, that the moving element 3 has come to a standstill, so thatthe moving element 3 is moved to a stable point. However, instead of thestable-point movement command unit 58, by using a stable-pointsimulation unit, which simulates the position of the moving element bythe stable point determined by the phase −3Δθ of the currents that hadbeen flowing in the moving element 3 when the reverse-movementdetermining unit 64 detected a reverse-movement, or by the phase −2Δθ ofthe currents that are one step prior to the currents upon the reversal,the magnetic-pole detecting system 1 may be configured more simply.

EMBODIMENT 2

Another embodiment of the present invention will be described, mainlyaccording to FIG. 4. The foregoing embodiment was described ignoring theinfluence of cogging torque that originates in the AC motor 5; however,in fact, sinusoidal cogging torque T_(c) occurs as shown in FIG. 4.Therefore, in the present embodiment, a highly accurate magnetic-poledetecting system that is unsusceptible to the cogging torque T_(c) isset forth.

It is known that the maximal value of the sinusoidal cogging torqueT_(c) occurs m times over a range of 2II in terms of electrical angle ofthe moving element 3, wherein m is the least-common multiple of thenumber of slots in the stator 2 and double the number of pole-pairs Pt.

In this situation, for example, if the stator 2 has 12 slots and 8poles, this least common multiple m is 24; therefore, positive ornegative peak values of the cogging torque T_(c) occur every 15 degrees,and the cogging torque T_(c) becomes zero every 7.5 degrees as given byelectrical angles θ1, θ2 (2θ1), θ3 (3θ1).

Here, the electrical angles θ1, θ2, θ3 may be generalized andrepresented as in the following relationship: electrical angleθ_(n)=n_(c)θ1 (n_(c): a positive integer).

Accordingly, the influence of the cogging torque can be lessened bysetting the phase nΔθ of the current command issued by the phase-updatecommand unit 52, as in the foregoing case for example, to 7.5 degrees asan electrical angle (phase) at which the cogging torque is diminished,or preferably is zero.

EMBODIMENT 3

Embodiments 1 and 2 above described the detection of positions of themoving element 3, ignoring the influence of friction. However, inreality, since friction exists when the moving element 3 moves, thestable point of the moving element 3 differs from its standstillposition. In order to reduce the influence of this type of friction, ameans is available for increasing the currents flowing in the coils 3 aand 3 b of the moving element 3 so that the attractive force of themoving element 3 is increased, but such a means is not appropriatesince, when the currents flowing are increased, the rated current ofsemiconductor devices forming the current generating unit 30 increases.

Therefore, a magnetic-pole detecting system is set forth according toFIG. 5, wherein the accuracy of detecting the magnetic-pole positions ofthe moving element 3 is raised without increasing the currents that flowin the coils 3 a and 3 b. FIG. 5 is a characteristic curve chartrepresenting the attractive torque TL and the static-friction torque TL′vs. the phase (distance), of the moving element 3.

In FIG. 5, assuming that the moving element 3 is at a standstill in theposition X_(s), the moving element 3 is moved in the forward directionby attractive forces F₀ and F₁; in contrast, under attractive force F₂,the moving element 3 remains still since no acceleration torque on themoving element 3 is caused.

In this situation, the first position X_(s1) (the first stable point),as a standstill starting point whereat the attractive force F₁ is zero,is obtained based on the phases nΔθ of the currents flowing in the coils3 a and 3 b that are necessary for the attractive force F₁ to occur. Inother words, in the above equation (3), assuming that the traveldistance ΔX_(max) is equal to the distance ΔX_(θ) in the phase Δθ, thefirst position X_(S1) of the moving element 3 is obtained from thefollowing equation:X _(s1)=τ(II/2−nΔθ)/2II   (8)

Next, as described in Embodiment 1, the phase Δθ of the currents issequentially updated by the phase-update command unit 52, so that themoving element 3 is reversed by the attractive force F₄. Based on thephase of the currents at this reversal, the second position X_(s2) (thesecond stable point) can be obtained from the above equation (8) in thesame way as the position X_(s1) described above. With the positionsX_(s1) and X_(s2) obtained in this way, the stable point X_(s) of themoving element 3 is approximately obtained from the following equation:X _(s) ≅X _(s0)=(X _(s1) +X _(s2))/2   (9)

The operation of detecting, by means of a magnetic-pole detectingsystem, the stable point X_(s0) that is the position of the movingelement 3, as described above, will be explained according to flowchartsin FIGS. 2, 5 and 6. In FIG. 6, numerical references identical to thosein FIG. 3 denote identical or corresponding operations, and theexplanation therefor will be omitted.

In the situation where the moving element 3 is at a standstill in theposition X_(s) in FIG. 5, the current generating unit 30, based on aforward-movement-current command signal, with phase −Δθ, from thephase-update command unit 52, makes currents flow in the coils 3 a and 3b by way of the current control unit 32 and 34 (Step S100); anattractive force F₁ occurs in the moving element 3, and the movingelement 3 is activated (Step S101); Steps S104 through S112 are carriedout, and n is set to 2 by adding 1 to the number of operations (StepS114).

The current generating section 30, by means of a forward-movementcurrent command signal from phase −2Δθ from the phase-update commandsection 52, causes currents to flow in the coils 3 a and 3 b by way ofthe current control units 32 and 34 (Step S100); an attractive force F₂acts on the moving element 3; however, the moving element 3 does notmove since the attractive force F₂ is less than the static-frictiontorque TL (Step S101). The moving element 3 comes to the firststandstill by means of the forward-movement-current command signal (StepS121); the CPU 24 obtains the first position X_(s1), according to theabove equation (8) with a phase −2Δθ, i.e., by executing the firstposition-calculating means; and stores it in the RAM 27 (Step S122). Thecurrent generating unit 30 cuts off the currents flowing based on acurrent cut-off command signal from the current cut-off command unit 54(Steps S124 and S104), when the currents continue to flow in the coils 3a and 3 b for more than a predetermined time period. Steps S106 throughS112 are carried out, and n is set to 3 by adding 1 to the number ofoperations (Step S114).

When the above Step S100 is carried out and the currents of phase −3Δθflow in the coils 3 a and 3 b (Step S100), the attractive force F₃occurs in the moving element 3; however, the moving element 3 remainsstill since the attractive force F₂ is less than the static-frictiontorque TL′ (Step S101). Since the halting of the moving element 3 is onebased on the current-phase update with the forward-movement-currentcommand signal, it is not the first halting (Step S121). Accordingly,Steps S124 and S104 through S112 are carried out, and n is set to 4 byadding 1 to the number of operations (Step S114).

When the above step S100 is carried out and currents of phase −4Δθ flowin the coils 3 a and 3 b (Step S100), the attractive force F₄, which islarger than the static-friction torque TL′, acts on the moving element3, so that the moving element 3 moves in reverse (Step S101). Thecurrents flowing in the coils 3 a and 3 b are cut off (Step S104); it isdetermined if the moving direction of the moving element 3 has reversed(Step S106); since it has reversed, the CPU 24, according to the aboveequation (8) with a phase −4Δθ, i.e., by executing the secondposition-calculating means, obtains the second position X_(s2) andstores it in the RAM 27 (Step S132); the stable point X_(s0), accordingto the above equation (9), is obtained (stable-point calculating means)and is regarded as the initial magnetic-pole position of the movingelement 3 (Step S134).

Although the above embodiments have described 2-phase synchronous ACmotors, the present invention can be applied also to N-phase AC motors.Moreover, although the above embodiments are applied to coil(winding)-moving types, they may also be applied to magnetic-pole-movingtypes.

INDUSTRIAL APPLICABILITY

As set forth heretofore, magnetic-pole detecting systems for synchronousAC motors and magnetic-pole detecting methods therefor according to thepresent invention are suited to applications for detecting initialmagnetic-pole positions in the synchronous AC motors.

1. A magnetic-pole detecting system for a synchronous AC motor,comprising: a synchronous AC motor having phase coils either in a movingelement or in a stator; a position-detecting means for generating aposition-detecting signal to detect positional relationship between themoving element and the stator; a movement-determining means forgenerating a movement signal upon determining that the moving elementhas moved, based on the position-detecting signal from theposition-detecting means; a current command generating means forgenerating a first current command signal for making a plurality ofdifferent-phase currents flow so that the moving element moves to aplurality of stable points, and for generating a second current commandsignal that makes currents flow, the currents having a phase that makesthe moving element move in reverse to the direction in which the movingelement has moved based on the first current command signal; a currentcontrolling means for making the currents flow in the phase coils, basedon the first and the second current command signals; a current cut-offmeans for cutting off the currents flowing in the each-phase coils,based on the movement signal from the movement-determining means; areversal-determining means for determining, based on a detection valuefrom the position-detecting means, the direction in which the movingelement has moved based on the first current command signal, and fordetermining that the detected direction has reversed between previousand present instances; and a stable-point simulating means forsimulating the position of the moving element by means of the stablepoint determined by the phase of the first current command signal whenthe reversal-determining means has detected a reversal, or by the phaseprior to the phase at the reversal.
 2. A magnetic-pole detecting systemfor a synchronous AC motor according to claim 1, wherein the currentphase of the first or the second current command signal is a phase atwhich sinusoidal cogging torque generated in the synchronous AC motor isapproximately zero.
 3. A magnetic-pole detecting system for asynchronous AC motor according to claim 1, further comprising: aphase-updating means for updating the phase of the first current commandsignal in steps of Δθ; a standstill-determining means for generating astandstill signal upon determining that the moving element has come to astandstill, based on the position-detecting signal from theposition-detecting means, after the currents are made to flow into thephase coils by the first current command signal; a firstposition-calculating means for obtaining a first position equal to thefirst stable point, based on the first phase of the first currentcommand signal upon the occurrence of the standstill signal; a secondposition-calculating means for obtaining a second position equal to thesecond stable point, based on the second phase of the first currentcommand signal when the reversal-determining means has made adetermination; and a stable-point calculating means for obtaining, basedon the first and the second positions, the position of the stable point;wherein the current controlling means maintains the phase of the movableelement when at a standstill, even when the currents are made to flow inthe phase coils, according to the first current command signal.
 4. Amagnetic-pole detecting system for a synchronous AC motor according toclaim 1, wherein the currents generated by the current controlling meanshave a time constant longer than that determined from the resistancecomponent and the inductance component of the AC motor.
 5. Amagnetic-pole detecting system for a synchronous AC motor according toclaim 1, further comprising: a cut-off signal generating means forgenerating a current cut-off command signal when a predetermined timeperiod elapses from the occurrence of the first or the second currentcommand signal; wherein the current cut-off means cuts off the currentsin phase coils, based on the current cut-off command signal from thecut-off signal generating means.
 6. A magnetic-pole detecting system fora synchronous AC motor according to claim 5, wherein the current phaseof the first or the second current command signal is a phase at whichsinusoidal cogging torque generated in the synchronous AC motor isapproximately zero.
 7. A magnetic-pole detecting system for asynchronous AC motor according to claim 5, further comprising: aphase-updating means for updating the phase of the first current commandsignal in steps of Δθ; a standstill-determining means for generating astandstill signal upon determining that the moving element has come to astandstill, based on the position-detecting signal from theposition-detecting means, after the currents are made to flow into thephase coils by the first current command signal; a firstposition-calculating means for obtaining a first position equal to thefirst stable point, based on the first phase of the first currentcommand signal upon the occurrence of the standstill signal; a secondposition-calculating means for obtaining a second position equal to thesecond stable point, based on the second phase of the first currentcommand signal when the reversal-determining means has made adetermination; and a stable-point calculating means for obtaining, basedon the first and- the second positions, the position of the stablepoint; wherein the current controlling means maintains the phase of themovable element when at a standstill, even when the currents are made toflow in the phase coils, according to the first current command signal.8. A magnetic-pole detecting system for a synchronous AC motor accordingto claim 5, wherein the currents generated by the current controllingmeans have a time constant longer than that determined from theresistance component and the inductance component of the AC motor.
 9. Amagnetic-pole detecting system for a synchronous AC motor, comprising: asynchronous AC motor having phase coils either in a moving element or ina stator; a position-detecting means for generating a position-detectingsignal to detect positional relationship between the moving element andthe stator; a movement-determining means for generating a movementsignal upon determining that the moving element has moved, based on theposition-detecting signal from the position-detecting means; a currentcommand generating means for generating a first current command signalfor making a plurality of different-phase currents flow so that themoving element moves to a plurality of stable points, and for generatinga second current command signal that makes currents flow, the currentshaving a phase that makes the moving element move in reverse to thedirection in which the moving element has moved based on the firstcurrent command signal; a current controlling means for making thecurrents flow in the phase coils, based on the first and the secondcurrent command signals; a current cut-off means for cutting off thecurrents flowing in the each-phase coils, based on the movement signalfrom the movement-determining means; a reversal-determining means fordetermining, based on a detection value from the position-detectingmeans, the direction in which the moving element has moved based on thefirst current command signal, and for determining that the detecteddirection has reversed between previous and present instances; and acurrent-maintaining means for inactivating the current cut-off meansbased on the determination, by the reversal-determining means, that thereversal has occurred, and for continuing to make the currents with thephase at the reversal or currents with the phase prior to the phase atthe reversal flow into the phase coils, until the moving element isdetermined, based on the detecting signal from the position-detectingmeans, to have come to a standstill.
 10. A magnetic-pole detectingsystem for a synchronous AC motor according to claim 9, wherein thecurrent phase of the first or the second current command signal is aphase at which sinusoidal cogging torque generated in the synchronous ACmotor is approximately zero.
 11. A magnetic-pole detecting system for asynchronous AC motor according to claim 9, further comprising: aphase-updating means for updating the phase of the first current commandsignal in steps of Δθ; a standstill-determining means for generating astandstill signal upon determining that the moving element has come to astandstill, based on the position-detecting signal from theposition-detecting means, after the currents are made to flow into thephase coils by the first current command signal; a firstposition-calculating means for obtaining a first position equal to thefirst stable point, based on the first phase of the first currentcommand signal upon the occurrence of the standstill signal; a secondposition-calculating means for obtaining a second position equal to thesecond stable point, based on the second phase of the first currentcommand signal when the reversal-determining means has made adetermination; and a stable-point calculating means for obtaining, basedon the first and the second positions, the position of the stable point;wherein the current controlling means maintains the phase of the movableelement when at a standstill, even when the currents are made to flow inthe phase coils, according to the first current command signal.
 12. Amagnetic-pole detecting system for a synchronous AC motor according toclaim 9, wherein the currents generated by the current controlling meanshave a time constant longer than that determined from the resistancecomponent and the inductance component of the AC motor.
 13. Amagnetic-pole position detecting method for a synchronous AC motorhaving phase coils either in a moving element or in a stator, aposition-detecting means for generating a position-detecting signal todetect positional relationship between the moving element and thestator, a current command generating means for generating a firstcurrent command signal that makes a plurality of different-phasecurrents flow-so that the moving element moves to a plurality of stablepoints, and for generating a second current command signal that makescurrents flow, the currents having a phase that makes the moving elementmove in reverse to the direction in which the moving element has movedbased on the first current command signal, a current controlling meansfor making the currents flow in the coils of each phase, based on thefirst and the second current command signals, the method comprising: afirst step of making, by means of the current control means, currentswith a first phase flow in the phase coils, so that the moving elementmoves, based on the first current command signal, to a first stablepoint; a second step of cutting off the currents by means of a currentcut-off means, when a movement-determining means detects, based on theposition-detecting signal, a movement of the moving element; a thirdstep of making, based on the second current command signal, currentswith a second phase flow in the phase coils, by means of the currentcontrolling means, so that the moving element reverses from a directionin which the moving element has moved; a fourth step of cutting off thecurrents by means of the current cut-off means when the movement of themoving element is detected by the movement-determining means; a fifthstep of changing the phase of the first current command signal to thephase for a second stable point that is different from the phase for thefirst stable point, and of making the currents flow in the phase coilsby means of the current controlling means; a sixth step of determining,based on the position-detecting signal from the position-detectingmeans, a direction in which the moving element moves based on the firstcurrent command signal, and of determining whether or not the detecteddirection has reversed between the previous and the present instances;wherein the first step through the sixth step are sequentially carriedout, and the stable point determined by the phase of the first currentcommand signal when the reversal-determining means has detected areversal, or by the phase prior to the phase at the reversal issimulated as the position of the moving element by a stable-pointsimulating means.
 14. A magnetic-pole position detecting method for asynchronous AC motor according to claim 13, wherein the current phase ofthe first or the second current command signal is a phase at whichsinusoidal cogging torque generated in the synchronous AC motor isapproximately zero.
 15. A magnetic-pole position detecting method for asynchronous AC motor according to claim 13, wherein the currentsgenerated by the current controlling means have a time constant longerthan that determined from the resistance component and the inductancecomponent of the AC motor.